1. Field of the Invention
The present invention relates to an optical detection device. It particularly applies to the detection of infrared radiation.
2. Discussion of the Background
Optical detection devices making it possible to detect light radiation in the near infrared range are already known. Such devices are e.g. referred to in the following documents:
Article by W. F. KOSONOCKY et al, published in Proceedings of the 14th Conference (1982 International) on Solid State Devices, Tokyo, 1982, Japanese Journal of Applied Physics, vol. 22, 1983, Supplement 22-1, pp 103-108. PA0 Article by BOR-YEU TSAUR et al, published by IEEE Electron Device Letters, vol. 9, Nos. 12, December 1988, pp 650-653. PA0 Article by BOR-YEU TSAUR et al, published in IEEE Electron Device Letters, vol. 10, No. 8, August 1989, pp. 361-363.
These known devices are a Schottky diodes comprising a thin metal film with a thickness of approximately 10 nm deposited on a semiconductor substrate.
The principle of optical detection by means of Schottky diode is diagrammatically illustrated by FIG. 1, in which it is possible to see the junction between the metal film 2, whose Fermi level is designated EF, and the semiconductor substrate 4, whose valence band and conduction band are respectively designated BV and BC.
The optical detection principle in this Schottky diode is that the incident light radiation constituted by h.f. energy photons is absorbed by the electrons of the metal film 2, h being the Planck constant and f the frequency of the photons.
When a e.sup.- electron has absorbed a photon, it then has an energy equal to the sum of the energy EI which it had prior to the absorption of the photon and the h.f. energy of the photon which it has absorbed. If this sum exceeds the height phi of the potential barrier existing at the interface between the metal film and the semiconductor substrate, the excited electron is liable to clear the potential barrier and can be collected by the semiconductor substrate.
This leads to an internal photoemission of electrons from the metal film to the semiconductor substrate. This internal photoemission causes the passage of a photocurrent, which can then be amplified and detected.
The energy range of the detected radiation is downwardly limited by the height phi of the Schottky barrier of the diode and which is equal to the difference between the bottom of the conduction band BC of the semiconductor substrate and the Fermi level EF of the metal film. This barrier height is determined by the chosen pair (metal, semiconductor).
For example, for type N silicon, the barrier height is approximately 0.3 to 0.95 eV as a function of the metal chosen.
Moreover, the potential barrier height is virtually independent of the voltage applied between the diode terminals, a very small phi variation of a few dozen meV at the most, being obtainable by applying a voltage of a few volts to the diode (Schottky effect).